Tio Pedro wrote:

I have to wonder if the guy took a broadband, untuned
PD circuit and tried to use it as a broadband mixer?

Almost certainly. He was therefore destined to fail completely!

If that's the case, then the interelectrode
capacities would definitely limit BW in a high
impedance circuit, unless they were
"tuned" out using a LC resonant circuit.

Exactly. "Tuning them out" would make the mixer somewhat less "broadband"!

I ran into that
headscratcher trying to figure out how to drive the
deflectors using a broadband balun type arrangement;
like was done in the Squires RX.

I looked at the Squires design, and it was very clever, but seemed to
require some obscure components that were unobtainable.

Bob